Pigmentary titanium dioxide is the most widely used white pigment in commerce today. Its preeminence is due, in the main, to its extraordinary combination of properties including little or no adsorption of visible light, high refractive index, high opacity and the ability to confer durability to coatings containing this pigment. However, regardless of the preeminence of this pigment much effort has been and continues to be expended to further improve its performance. This particularly is true with regard to efforts to reduce the pigment's tendency to adsorb moisture and to improve its gloss developing and dispersibility characteristics.
Many proposals have been made in the art for improving the gloss developing and dipersibility characteristics of pigmentary titanium dioxide. In general, these proposals have involved the treatment of the pigment with a diversity of organic substances of various descriptions. For example, U.S. Pat. No. 3,015,573 discloses the treatment of dry titanium dioxide pigments with amine salts prepared from water soluble tertiary amines such as triethanolamine with sparingly water soluble organic acids such as adipic acid. According to this patent, the use of sparingly water soluble organic acids, i.e., acids having water solubilities in the range of from 0.05 to 2.0 weight percent, to prepare the amine salt treating agents is essential if pigments that are more readily dispersible in both organic and aqueous media are to be obtained, a major objective of the invention of this patent. In addition to being more readily dispersible, the pigments described in this patent are said to exhibit a greater tendency to dry flow and to form as few as 20 percent of the oversize aggregates which said pigment would otherwise form. The other principal physical properties, e.g., oil adsorption, color, gloss and the like are substantially unaffected and remain substantially those of the corresponding pigments without the amine salt.
In a similar manner, U.S. Pat. No. 3,506,466 discloses the chemical treatment of pigmentary titanium dioxide to improve its dispersibility. According to this patent, amine salts prepared from alkanolamines such as triethanolamine, triisopropanolamine, and the like and oxycarboxylic acids such as citric acid, tartaric acid, and the like are capable of improving the dispersibility of pigmentary titanium dioxide as well as other properties thereof including tinting strength and scattering coefficient. The amine salts are applied either in pure form or dissolved in water and added to an aqueous slurry of the pigment. The slurry then is dried and the dried pigment subjected to fluid energy milling.
U.S. Pat. No. 3,825,438 discloses the coating of titanium dioxide pigment with one or more hydrous metal oxides in the presence of a polyhydric alcohol and/or a carboxylic acid. The alcohols and/or the acids useful in the practice of the invention of this patent are those that contain at least two hydroxyl groups, which term is defined to include the hydroxyl groups present in the carboxyl groups of the acids. In accordance with the teachings of this patent, while the alcohols and/or acids can be added to the titanium dioxide pigment at any convenient stage, it is essential that such addition be accomplished prior to completion of the deposition of the hydrous metal coating. The resulting pigment is said to have improved dispersibility when compared with a pigment coated in the absence of these alcohols and/or acids. In addition, such treatment is said to further improve one or more of the gloss, hiding power, stoving color and drying time of the pigment produced using this treatment.
A treatment specifically directed to improving the gloss developing properties of titanium dioxide pigment is disclosed in U.S. Pat. No. 3,173,722. According to the disclosure in this patent, the gloss developing properties of titanium dioxide pigments are improved by treating a wet milled hydrous metal oxide coated titanium dioxide pigment with a combination of either benzoic or para-aminobenzoic acid and an amine such as triethanolamine. The treatment merely comprises adding the acid and amine to the pigment and, without removing these materials from the pigment, milling the pigment to produce a finely divided, high tinting strength material having greatly improved gloss developing properties.
It is evident from the above art disclosure that many different chemical treatments have been proposed to improve various physical properties of titanium dioxide pigments. This is particularly true with respect to improving the dispersibility and dry flow characteristics of titanium dioxide pigment. However, many of the chemical treatments proposed would appear to be specific for improving only that property of the pigment which it is desired to improve. At best, other properties remain substantially the same as the corresponding untreated pigment or, at worse, are adversely effected by the treatment.
The present invention is directed principally to the chemical treatment of titanium dioxide pigments to improve the dispersibility of the pigment in surface coating vehicles. Surprisingly, however, the pigments so treated further exhibit significantly improved gloss developing properties and reduced tendencies to adsorb moisture.